The plastic production process begins by heating the hydrocarbons in a “cracking process.” Here, in the presence of a catalyst, larger molecules are broken down into smaller ones such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and other hydrocarbons. The yield of ethylene is controlled by the cracking temperature and is more than 30% at 850° C. and such products as styrene and vinylchloride can be produced in subsequent reactions. These are the starting materials for several other types of plastics. Therefore, this process results in the conversion of the natural gas or crude oil components into monomers such as ethylene, propylene, butene and styrene.
These monomers are then polymerized. Different combinations of monomers yield plastic resins with different properties and characteristics. Each monomer yields a plastic resin with different properties and characteristics. Combinations of monomers produce copolymers with further property variations.
The resulting resins may be molded or formed to produce several different kinds of plastic products with application in many major markets. The variability of resin permits a compound to be tailored to a specific design or performance requirement. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. For instance, impact strength measures the ability of a material to withstand shock loading. Heat resistance protects the resin from exposure to excessive temperatures. Chemical resistance protects the resin from breakdown due to exposure to environmental chemicals.
Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous, and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone and these are considered inorganic polymers.
Polymers produced on a large scale such as polystyrene, polyvinyl chloride (PVC), polyethylene co-vinylacetate, polyacrylic acid, and styrene and butadiene rubber, are created using heterogeneous dispersion polymerization. In this process, polymers form in two phases in which the initial monomer or the resulting polymer, or both, are finely dispersed in a solvent and are controlled by adding a surfactant (stabilizer) that conforms particle sizes to within a relatively narrow range. Once the polymers are formed, manufacturers remove the water or evaporate the solvents used to disperse the polymers. Companies then face the challenge of properly disposing of and remediating these harmful by-products, a daunting task as EPA regulations grow more stringent. In 1992, the U.S. plastics industry produced 567 million pounds of toxic waste, according to the EPA. In the same year, companies put about one-quarter of their total waste back into the nation's environment.
It is generally desirable to recycle/re-use plastics materials. Aside from preserving resources, significant costs and taxes can be incurred in dumping such materials.
Plastic is more than just a landfill problem. The more than 75 billion pounds of plastic products and packaging produced in the United States every year poses a wide variety of dangers to human health and the environment. At every step in the production of plastics, hazardous substances are used and hazardous wastes are produced. Plastics are made from finite, nonrenewable petroleum and natural gas. Production of plastic products and packaging is one of the most chemically intensive activities. According to the U.S. Environmental Protection Agency (EPA), 35 of the 47 chemical plants ranked highest in carcinogenic emissions are involved in plastic production. Workers at these chemical refineries (along with nearby residents), are at increased risk of injury or death due to toxic emissions and/or chemical explosions. Plastics contain additives (i.e. colorants, stabilizers, and plasticizers) that may include toxic constituents such as cadmium and lead. Some plastic chemicals, such as ethylene dichloride and vinyl chloride used to produce vinyl are considered to be carcinogenic. They may also trigger other health problems such as liver, kidney and neurological damage. Chemicals in plastics may reduce sperm counts. A panel convened by the National Institutes of Health found that the most commonly used plasticizer, DEHP, was a developmental toxin. Studies showed that male rodents exposed to DEHP had decreased sperm levels. DEHP, which is used in plastic food packaging, children's toys and medical devices, has the potential to leach out of plastics. Exposure can occur through breathing, ingestion and possibly through absorption of the skin.
Plastic litter and waste represents a significant and growing cost to the state, local government and ultimately ratepayers and taxpayers. California's annual garbage bill for cleaning up and landfilling plastic waste is conservatively estimated at more than $750 million annually. Studies show that plastic represents 50 to 80 percent of the volume of litter collected from roads, parks and beaches, and 90 percent of floating litter in the marine environment. State and local agencies spend millions picking up litter each year, of which plastic is often the largest component. Last year California Transportation alone spent $16 million cleaning up litter on California highways. This sum excludes the costs incurred by volunteers and businesses participating in the Adopt-a-Highway program. California's total cleanup cost for plastic litter and waste cleanup will easily top one billion dollars in 2004.
Of hundreds of varieties of plastic, just six are used in 60 percent of plastic production, including at least 90 percent of plastic packages and containers. Polyethylene terephthalate (PET or PETE) is found in beverage, mouthwash, peanut butter and salad dressing bottles; in TV dinner trays and plastic wrap. Recycled PETE is used to make clothing fibers, tote bags, furniture, carpet and new beverage containers. High density polyethylene (HDPE) is found in milk, water and juice bottles, trash and shopping bags; and in detergent, yogurt and margarine containers. Recycled HDPE is used to make detergent, oil and vitamin bottles, drain pipes, recycling bins, dog houses, and other plastic lumber. Polyvinyl chloride (PVC) is used to make clear food packages, shampoo bottles, toiletry and medicine tubes. Recycled PVC is used to make packaging, binders, mats, decks, paneling, roadway gutters, mud flaps and speed bumps. Low density Polyethylene (LDPE) is used to make bread bags, frozen food bags and squeezable bottles. Recycled LDPE is used to make mailing envelopes, trash cans, trash liners, furniture, floor tiles, paneling and compost bins and lumber. Polypropylene (PP) is used in ketchup, medicine and dairy product containers. Recycled PP is used to make signal lights, car battery components, brooms, brushes, oil funnels, ice scrapers, bike racks, pallets, storage bins and trays. Polystyrene (PS) is used to make packing foam, egg cartons, meat trays, aspirin bottles, plates, CD jackets and food service items. Recycled PS is used to make thermometers, switch plates, insulation, egg cartons, vents, office supplies, foam packaging and containers.
Of these six plastics, there are widespread recycling opportunities for just two: PET and HDPE. Together, those resins accounted for more than 94 percent of all the plastic bottles made in the United States in 1998. But even with these, there are limits to recycling opportunities. Most plastics cannot be economically retrieved in sufficient quantities to support a recycling market. Because of their varying physical and chemical properties, different plastics generally cannot be mixed together when they are reprocessed without damaging them. Many residential recycling programs can only accept PET and HDPE beverage containers. Frozen food trays, margarine tubs and other food containers come in too many colors to be cost effectively collected in most communities.
A number of problems can be encountered in attempting to recycle/re-use plastics material. Such waste material tends to have a very high relative volume and hence can incur significant transport costs. Problems can also be encountered with contaminants and also mixtures of different materials. Problems can also be encountered with disposing of various types of waste which include plastics material such as used syringes and other “red waste” from hospitals and other medical establishments. Such materials must clearly be disposed of safely and without risk to persons handling them.